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rfc:rfc8539

Internet Engineering Task Force (IETF) I. Farrer Request for Comments: 8539 Deutsche Telekom AG Updates: 7598 Q. Sun Category: Standards Track Y. Cui ISSN: 2070-1721 L. Sun

                                                   Tsinghua University
                                                            March 2019
           Softwire Provisioning Using DHCPv4 over DHCPv6

Abstract

 DHCPv4 over DHCPv6 (RFC 7341) is a mechanism for dynamically
 configuring IPv4 for use as an over-the-top service in an IPv6-only
 network.  Softwires are an example of such a service.  For DHCPv4
 over DHCPv6 (DHCP 4o6) to function with some IPv4-over-IPv6 softwire
 mechanisms and deployment scenarios (e.g., RFC 7596 or RFC 7597), the
 operator needs to know the IPv6 address that the client will use as
 the source of an IPv4-in-IPv6 softwire tunnel.  This address, in
 conjunction with the client's IPv4 address, and (in some deployments)
 the Port Set ID are used to create a binding table entry in the
 operator's softwire tunnel concentrator.  This memo defines a DHCPv6
 option to convey IPv6 parameters for establishing the softwire tunnel
 and a DHCPv4 option (to be used only with DHCP 4o6) to communicate
 the source tunnel IPv6 address between the DHCP 4o6 client and
 server.  It is designed to work in conjunction with the IPv4 address
 allocation process.
 "DHCPv6 Options for Configuration of Softwire Address and Port-Mapped
 Clients" (RFC 7598) describes a deterministic DHCPv6-based mechanism
 for provisioning softwires.  This document updates RFC 7598, allowing
 OPTION_S46_BR (90) to be enumerated in the DHCPv6 client's Option
 Request Option (ORO) request and to appear directly within subsequent
 messages sent by the DHCPv6 server.

Farrer, et al. Standards Track [Page 1] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8539.

Copyright Notice

 Copyright (c) 2019 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Farrer, et al. Standards Track [Page 2] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
 4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . .   5
   4.1.  Updating RFC 7598 to Permit the Reuse of
         OPTION_S46_BR (90)  . . . . . . . . . . . . . . . . . . .   5
 5.  DHCP 4o6 IPv6/IPv4 Binding Message Flow . . . . . . . . . . .   6
 6.  DHCP Options  . . . . . . . . . . . . . . . . . . . . . . . .   7
   6.1.  DHCPv6 Softwire Source Binding Prefix Hint Option . . . .   7
   6.2.  DHCP 4o6 Softwire Source Address Option . . . . . . . . .   8
 7.  Client Behavior . . . . . . . . . . . . . . . . . . . . . . .   9
   7.1.  Client Initialization . . . . . . . . . . . . . . . . . .   9
   7.2.  Renewing or Rebinding the IPv4 Address Lease and
         Softwire Source Address . . . . . . . . . . . . . . . . .  10
     7.2.1.  Changing the Bound IPv6 Softwire Source Address . . .  10
   7.3.  Releasing the IPv4 Address Lease and Softwire
         Source Address  . . . . . . . . . . . . . . . . . . . . .  11
   7.4.  OPTION_S46_BIND_IPV6_PREFIX Validation Behavior . . . . .  11
   7.5.  Client and Server Softwire Source Address Mismatch  . . .  11
   7.6.  Use with Dynamic, Shared IPv4 Addresses . . . . . . . . .  12
 8.  Server Behavior . . . . . . . . . . . . . . . . . . . . . . .  12
   8.1.  Changing the Bound IPv6 Source Address  . . . . . . . . .  12
   8.2.  Handling Conflicts between Clients' Bound IPv6 Source
         Addresses . . . . . . . . . . . . . . . . . . . . . . . .  12
 9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.1.  Client Privacy Considerations . . . . . . . . . . . . . .  14
 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
   11.2.  Informative References . . . . . . . . . . . . . . . . .  17
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1. Introduction

 Deterministic IPv4-over-IPv6 transition technologies require that
 elements be preconfigured with binding rules for routing traffic to
 clients.  This places a constraint on the choice of address used as
 the client's softwire source address: it must use a predetermined
 prefix, which is usually configured on the home gateway device.
 [RFC7598] describes a DHCPv6-based mechanism for provisioning such
 deterministic softwires.

Farrer, et al. Standards Track [Page 3] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 A dynamic provisioning model, such as using DHCPv4 over DHCPv6 (DHCP
 4o6) [RFC7341], allows much more flexibility in the location of the
 IPv4-over-IPv6 softwire source address.  In this model, the IPv6
 address is dynamically communicated back to the service provider,
 allowing the corresponding softwire configuration to be created in
 the border relay (BR).
 The DHCP 4o6 client and softwire client could be run on end devices
 attached to a network segment using any routable IPv6 prefix
 allocated to an end user, located anywhere within an arbitrary home
 network topology.  Dynamic allocation also helps to optimize IPv4
 resource usage, because only clients that are actively renewing their
 IPv4 lease hold on to the address.
 This document describes a mechanism for dynamically provisioning
 softwires created using DHCP 4o6, including provisioning the client
 with the address of the softwire BR and informing the service
 provider of a client's binding between the dynamically allocated IPv4
 address and Port Set ID and the IPv6 address that the softwire
 initiator will use for accessing IPv4-over-IPv6 services.
 The mechanism operates alongside the DHCP 4o6 message flows to
 communicate the binding information over the IPv6-only network.  The
 DHCP 4o6 server provides a single point in the network that holds the
 current client binding information.  The service provider can then
 use this binding information to provision other functional elements,
 such as the BR(s).

2. Applicability

 The mechanism described in this document is only suitable for use for
 provisioning softwire clients via DHCP 4o6.  The options described
 here are only applicable within the DHCP 4o6 message-exchange
 process.  Current softwire technologies suitable for extending to
 incorporate DHCP 4o6 with dynamic IPv4 address leasing include
 [RFC7597] and [RFC7596].

3. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

Farrer, et al. Standards Track [Page 4] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

4. Solution Overview

 In order to provision a softwire, both IPv6 and IPv4 configurations
 need to be passed to the client.  To map this to the DHCP 4o6
 configuration process, the IPv6 configuration is carried in DHCPv6
 options [RFC8415], carried inside the DHCPv6 message DHCPV4-RESPONSE
 (21) sent by the server.  OPTION_S46_BR (90) is used to provision the
 remote IPv6 address for the softwire BR (see Section 4.1).
 OPTION_S46_BIND_IPV6_PREFIX (137) is optionally sent by the DHCP 4o6
 server to indicate to the client a preferred IPv6 prefix for binding
 the received IPv4 configuration and sourcing tunnel traffic.  This
 may be necessary if there are multiple IPv6 prefixes in use in the
 customer network (e.g., Unique Local Addresses (ULAs)) or if the
 specific IPv4-over-IPv6 transition mechanism requires the use of a
 particular prefix for any reason.
 IPv4 configuration is carried in DHCPv4 messages [RFC2131] (inside
 the DHCP 4o6 option OPTION_DHCPV4_MSG (87)) using the mechanism
 described in [RFC7341].
 In order for the client to communicate the softwire source address, a
 new DHCPv4 option OPTION_DHCP4O6_S46_SADDR (109) is defined in this
 document.  This is included in DHCPREQUEST messages sent by the
 client and is stored by the server for the lifetime of the IPv4
 address lease.

4.1. Updating RFC 7598 to Permit the Reuse of OPTION_S46_BR (90)

 Section 4.2 of [RFC7598] defines option OPTION_S46_BR (90) for
 communicating remote softwire BR IPv6 address(es) to a client, but it
 mandates that the option can only be used when encapsulated within
 one of the softwire container options: OPTION_S46_CONT_MAPE (94) or
 OPTION_S46_CONT_LW (96).  From Section 3 of [RFC7598]:
    Softwire46 DHCPv6 clients that receive provisioning options that
    are not encapsulated in container options MUST silently ignore
    these options.
 This document updates [RFC7598], removing this restriction for
 OPTION_S46_BR (90), allowing it to be enumerated in the client's ORO
 request and appear directly within subsequent messages sent by the
 DHCPv6 server.

Farrer, et al. Standards Track [Page 5] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

5. DHCP 4o6 IPv6/IPv4 Binding Message Flow

 Figure 1 shows the relevant extensions to the successful DHCP 4o6
 IPv4 allocation client/server message flow for the softwire source
 address function.  The full process, including error handling, is
 described in Section 7.
 In each step, the DHCPv6 portion of the message and any relevant
 option is shown above the arrow.  The DHCP 4o6 content of the message
 and its relevant options are below the arrow.  All the DHCPv4
 messages are encapsulated in DHCPV4-QUERY (20) or DHCPV4-RESPONSE
 (21) messages.  Where relevant, the necessary options and their
 contents are shown.
      DHCP 4o6                                              DHCP 4o6
       Client                                                Server
         |                                                      |
         |       DHCPv6 - DHCPV4-QUERY message containing       |
         |           OPTION_ORO (6) listing (90, 137)           |
  Step 1 |----------------------------------------------------->|
         |            DHCPv4 - DHCPDISCOVER message             |
         |                                                      |
         |                                                      |
         |     DHCPv6 - DHCPV4-RESPONSE message containing      |
         | OPTION_S46_BR(90), OPTION_S46_BIND_IPV6_PREFIX(137)  |
         |     (bind-ipv6-prefix with service provider's        |
         |                  preferred prefix)                   |
  Step 2 |<-----------------------------------------------------|
         |              DHCPv4 - DHCPOFFER message              |
         |         containing an available IPv4 address         |
         |                                                      |
         |             DHCPv6 - DHCPV4-QUERY message            |
  Step 3 |----------------------------------------------------->|
         |     DHCPv4 - DHCPREQUEST message containing the      |
         | requested IPv4 address and OPTION_DHCP4O6_S46_SADDR  |
         |   (softwire-ipv6-src-address with client's bound     |
         |            IPv6 softwire source address)             |
         |                                                      |
         |                                                      |
         |           DHCPv6 - DHCPV4-RESPONSE message           |
  Step 4 |<-----------------------------------------------------|
         |          DHCPv4 - DHCPACK message containing         |
         | the leased IPv4 address and OPTION_DHCP4O6_S46_SADDR |
         |    (softwire-ipv6-src-address with client's bound    |
         |              IPv6 softwire source address)           |
         |                                                      |
               Figure 1: IPv6/IPv4 Binding Message Flow

Farrer, et al. Standards Track [Page 6] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 Step 1  The client constructs a DHCPv6 "DHCPV4-QUERY (20)" message.
         This message contains two options: DHCPv6 OPTION_ORO (6) and
         OPTION_DHCPV4_MSG (87).  OPTION_ORO lists "90"
         (OPTION_S46_BR) and "137" (OPTION_S46_BIND_IPV6_PREFIX).
         OPTION_DHCPV4_MSG contains a DHCPv4 DHCPDISCOVER message.
 Step 2  The server responds with a DHCPv6 "DHCPV4-RESPONSE (21)"
         message.  This message contains an OPTION_S46_BR (90)
         containing the IPv6 address of the BR for the client's
         softwire configuration.  The message may also optionally
         contain OPTION_S46_BIND_IPV6_PREFIX (137).  OPTION_DHCPV4_MSG
         contains a DHCPv4 DHCPOFFER message.  The DHCPv4 message
         contains an available IPv4 address.
 Step 3  The client sends a DHCPv6 "DHCPV4-QUERY (20)" message
         containing a DHCPv4 DHCPREQUEST message with the requested
         IPv4 address and OPTION_DHCP4O6_S46_SADDR (109) with the IPv6
         address that the client will use as its softwire source
         address.
 Step 4  The server sends a DHCPv6 "DHCPV4-RESPONSE (21)" message.
         OPTION_DHCPV4_MSG contains a DHCPv4 DHCPACK message with the
         allocated IPv4 address.  OPTION_DHCP4O6_S46_SADDR with the
         client's bound softwire source address is included.

6. DHCP Options

6.1. DHCPv6 Softwire Source Binding Prefix Hint Option

 The format of the DHCPv6 source binding prefix hint option is as
 follows:
    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  OPTION_S46_BIND_IPV6_PREFIX  |         option-length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |bindprefix6-len|                                               |
   +-+-+-+-+-+-+-+-+             bind-ipv6-prefix                  .
   .                            (variable length)                  .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            Figure 2: Format of OPTION_S46_BIND_IPV6_PREFIX
 o  option-code: OPTION_S46_BIND_IPV6_PREFIX (137)
 o  option-length: 1 + length of bind-ipv6-prefix, specified in bytes.

Farrer, et al. Standards Track [Page 7] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 o  bindprefix6-len: 8-bit field expressing the bit mask length of the
    IPv6 prefix specified in bind-ipv6-prefix.  Valid values are 0 to
    128.
 o  bind-ipv6-prefix: The IPv6 prefix indicating the preferred prefix
    for the client to bind the received IPv4 configuration to.  The
    length is (bindprefix6-len + 7) / 8.  The field is padded on the
    right with zero bits up to the next octet boundary when
    bind-ipv6-prefix is not evenly divisible by 8.  These padding bits
    are ignored by the receiver (see Section 7.4).
 OPTION_S46_BIND_IPV6_PREFIX is a singleton.  Servers MUST NOT send
 more than one instance of the OPTION_S46_BIND_IPV6_PREFIX option.

6.2. DHCP 4o6 Softwire Source Address Option

 The format of the DHCPv4 over DHCPv6 softwire source address option
 is as follows:
            0                             1
            0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           |      option-code      |     option-length     |
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
           +           softwire-ipv6-src-address           +
           .                  (128 bits)                   .
           +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             Figure 3: Format of OPTION_DHCP4O6_S46_SADDR
 o  option-code: OPTION_DHCP4O6_S46_SADDR (109)
 o  option-length: 16.
 o  softwire-ipv6-src-address: 16 bytes long; the IPv6 address that is
    associated (either being requested for binding or currently bound)
    with the client's IPv4 configuration.
 Note: The function of OPTION_DHCP4O6_S46_SADDR may seem similar to
 the DHCPv4 message's "chaddr" field or the Client Identifier (61)
 option in that it provides a unique lower-layer address that the
 server can use for identifying the client.  However, as both of these
 are required to remain constant throughout the address lease
 lifetime, they cannot be used with the mechanism described in this
 document.  This is because the client may only be able to construct
 the IPv6 address to use as the source address after it has received
 the first DHCPV4-RESPONSE message from the server containing
 OPTION_S46_BIND_IPV6_PREFIX.

Farrer, et al. Standards Track [Page 8] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

7. Client Behavior

 A client requiring dynamic softwire configuration first enables DHCP
 4o6 configuration using the method described in Section 5 of
 [RFC7341].  If OPTION_DHCP4_O_DHCP6_SERVER is received in the
 corresponding REPLY message, the client MAY continue with the
 configuration process described below.
 Before the dynamic softwire configuration process can commence, the
 client MUST be configured with a suitable IPv6 prefix to be used as
 the local softwire endpoint.  This could be obtained using DHCPv6,
 Router Advertisement (RA) / Prefix Information Option (PIO), or
 another mechanism.

7.1. Client Initialization

 When constructing the initial DHCP 4o6 DHCPDISCOVER message, the
 client includes a DHCPv6 OPTION_ORO (6) within the options field of
 the DHCP-QUERY message.  OPTION_ORO contains the option codes for
 OPTION_S46_BR (90) and OPTION_S46_BIND_IPV6_PREFIX (137).
 On receipt of the DHCP 4o6 server's reply (a DHCPV4-RESPONSE
 containing a DHCPOFFER message), the client checks the contents of
 the DHCPv4-RESPONSE for the presence of a valid OPTION_S46_BR option.
 If this option is not present, or does not contain at least one valid
 IPv6 address for a BR, then the client MUST discard the message, as
 without the address of the BR the client cannot configure the
 softwire and so has no interface to request IPv4 configuration for.
 The DHCPV4-RESPONSE message may also include
 OPTION_S46_BIND_IPV6_PREFIX, which is used by the operator to
 indicate a preferred prefix that the client should bind IPv4
 configuration to.  If received, the client first checks the option
 according to Section 7.4.  If valid, the client uses this prefix as
 the "IPv6 binding prefix" and follows to the process described in
 Section 5.1 of [RFC7596] in order to select an active IPv6 prefix to
 construct the softwire.  If no match is found, or the client doesn't
 receive OPTION_S46_BIND_IPV6_PREFIX, the client MAY select any valid
 IPv6 prefix (of a suitable scope) to use as the tunnel source.
 Once the client has selected a suitable prefix, it MAY either use an
 existing IPv6 address that is already configured on an interface or
 create a new address specifically for use as the softwire source
 address (e.g., using an Interface Identifier constructed as per
 Section 6 of [RFC7597]).  If a new address is being created, the
 client MUST complete configuration of the new address, performing
 duplicate address detection (if required) before proceeding.

Farrer, et al. Standards Track [Page 9] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 The client then constructs a DHCPV4-QUERY message containing a DHCPv4
 DHCPREQUEST message.  OPTION_DHCP4O6_S46_SADDR is included in the
 options field of the DHCPREQUEST message with the IPv6 address of its
 softwire source address in the softwire-ipv6-src-address field.
 When the client receives a DHCPv4 DHCPACK message from the server, it
 checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
 active softwire source address.  If they match, the allocation
 process has concluded.  If there is a discrepancy, then the process
 described in Section 7.5 is followed.
 If the client receives a DHCPv4 DHCPNAK message from the server, then
 the configuration process has been unsuccessful.  The client then
 restarts the process from Step 1 of Figure 1.

7.2. Renewing or Rebinding the IPv4 Address Lease and Softwire Source

    Address
 Whenever the client attempts to extend the lease time of the IPv4
 address, OPTION_DHCP4O6_S46_SADDR with the IPv6 address of its
 softwire source address in the softwire-ipv6-src-address field MUST
 be included in the DHCPREQUEST message.

7.2.1. Changing the Bound IPv6 Softwire Source Address

 Across the lifetime of the leased IPv4 address, it is possible that
 the client's IPv6 address will change, e.g., if there is an IPv6
 renumbering event.
 In this situation, the client MUST inform the server of the new
 address.  This is done by sending a DHCPREQUEST message containing
 OPTION_DHCP4O6_S46_SADDR with the new IPv6 source address.
 When the client receives a DHCPv4 DHCPACK message from the server, it
 checks the IPv6 address in OPTION_DHCP4O6_S46_SADDR against its
 active softwire source address.  If they match, the allocation
 process has concluded.  If there is a discrepancy, then the process
 described in Section 7.5 is followed.
 If the client receives a DHCPv4 DHCPNAK message in response from the
 server, then the change of the bound IPv6 softwire source address has
 been unsuccessful.  In this case, the client MUST stop using the new
 IPv6 source address.  The client then restarts the process from Step
 1 of Figure 1.

Farrer, et al. Standards Track [Page 10] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

7.3. Releasing the IPv4 Address Lease and Softwire Source Address

 When the client no longer requires the IPv4 resource, it sends a
 DHCPv4 DHCPRELEASE message to the server.  As the options field is
 unused in this message type, OPTION_DHCP4O6_S46_SADDR is not
 included.

7.4. OPTION_S46_BIND_IPV6_PREFIX Validation Behavior

 On receipt of the OPTION_S46_BIND_IPV6_PREFIX option, the client
 makes the following validation checks:
 o  The received bindprefix6-len value is not larger than 128.
 o  The number of bytes received in the bind-ipv6-prefix field is
    consistent with the received bindprefix6-len value (calculated as
    described in Section 6.1).
 If either check fails, the receiver discards the invalid option and
 proceeds to attempt configuration as if the option had not been
 received.
 The receiver MUST only use bits from the bind-ipv6-prefix field up to
 the value specified in the bindprefix6-len when performing the
 longest prefix match. bind-ipv6-prefix bits beyond this value MUST be
 ignored.

7.5. Client and Server Softwire Source Address Mismatch

 If the client receives a DHCPACK message with an
 OPTION_DHCP4O6_S46_SADDR containing an IPv6 address that differs from
 its active softwire source address, the client SHOULD wait for a
 randomized time interval and then resend the DHCPREQUEST message with
 the correct softwire source address.  Section 4.1 of [RFC2131]
 describes the retransmission backoff interval process.
 The default minimum time for the client to attempt retransmission is
 60 seconds.  If, after this time has expired, the client has not
 received a DHCPACK message with the correct bound IPv6 address,
 client MAY send a DHCPRELEASE message and restart the process
 described in Section 7.  The retry interval should be configurable
 and aligned with any server policy defining the minimum time interval
 for client address updates as described in Section 8.1.

Farrer, et al. Standards Track [Page 11] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

7.6. Use with Dynamic, Shared IPv4 Addresses

 [RFC7618] describes a mechanism for using DHCPv4 to distribute
 dynamic, shared IPv4 addresses to clients.  The mechanism described
 in this document is compatible with IPv4 address sharing and can be
 enabled by following the process described in Section 6 of [RFC7618].

8. Server Behavior

 Beyond the normal DHCP 4o6 functionality defined in [RFC7341], the
 server MUST also store the IPv6 softwire source address of the client
 in the leasing address database, alongside the IPv4 address and
 client identifier.
 An OPTION_DHCP4O6_S46_SADDR containing the bound softwire source
 address MUST be sent in every DHCPACK message sent by the server.
 The binding entry between the client's IPv6 softwire source address
 and the leased IPv4 address is valid as long as the IPv4 lease
 remains valid.

8.1. Changing the Bound IPv6 Source Address

 In the event that the server receives a DHCPREQUEST message for an
 active IPv4 lease containing an OPTION_DHCP4O6_S46_SADDR with an IPv6
 address that differs from the address that is currently stored, the
 server updates the stored softwire source address with the new
 address supplied by the client and sends a DHCPACK message containing
 the updated softwire source address in OPTION_DHCP4O6_S46_SADDR.
 The server MAY implement a policy enforcing a minimum time interval
 between a client updating its softwire source IPv6 address.  If a
 client attempts to update the softwire source IPv6 address before the
 minimum time has expired, the server can either silently drop the
 client's message or send back a DHCPACK message containing the
 existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.  If
 implemented, the default minimum client source address update
 interval is 60 seconds.

8.2. Handling Conflicts between Clients' Bound IPv6 Source Addresses

 In order for traffic to be forwarded correctly, each customer edge's
 (CE's) softwire IPv6 source address must be unique.  To ensure this,
 on receipt of every client DHCPREQUEST message containing
 OPTION_DHCP4O6_S46_SADDR, the DHCP 4o6 server MUST check the received
 IPv6 address against all existing CE source addresses stored for

Farrer, et al. Standards Track [Page 12] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 active client IPv4 leases.  If there is a match for any active lease
 other than the lease belonging to the client sending the DHCPREQUEST,
 then the client's IPv6 source address MUST NOT be stored or updated.
 Depending on where the client and server are in the address leasing
 lifecycle, the DHCP 4o6 server then takes the following action:
 o  If the DHCP 4o6 does not have a current, active IPv4 address lease
    for the client, then the DHCP address allocation process has not
    been successful.  The server returns a DHCPNAK message to the
    client.
 o  If the DHCP 4o6 does have a current, active IPv4 address lease,
    then the source address update process (see Section 8.1) has not
    been successful.  The DHCP 4o6 server can either silently drop the
    client's message or return a DHCPACK message containing the
    existing IPv6 address binding in OPTION_DHCP4O6_S46_SADDR.

9. Security Considerations

 Security considerations that are applicable to [RFC7341] are also
 applicable here.
 A rogue client could attempt to use the mechanism described in
 Section 7.2.1 to redirect IPv4 traffic intended for another client to
 itself.  This would be performed by sending a DHCPREQUEST message for
 another client's active IPv4 lease containing the attacker's softwire
 IPv6 address in OPTION_DHCP4O6_S46_SADDR.
 For such an attack to be effective, the attacker would need to know
 both the client identifier and the active IPv4 address lease
 currently in use by another client.  This could be attempted in three
 ways:
 1.  One customer learning the active IPv4 address lease and client
     identifier of another customer via snooping the DHCP4o6 message
     flow between the client and server.  The mechanism described in
     this document is intended for use in a typical ISP network
     topology with a dedicated Layer 2 access network per client,
     meaning that snooping of another client's traffic is not
     possible.  If the access network is a shared medium, then
     provisioning softwire clients using dynamic DHCP4o6 as described
     here is NOT RECOMMENDED.

Farrer, et al. Standards Track [Page 13] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

 2.  Learning the active IPv4 address lease and client identifier via
     snooping the DHCP4o6 message flow between the client and server
     in the aggregation or core ISP network.  In this case, the
     attacker requires a level of access to the ISP's infrastructure
     that means they can already intercept or interfere with traffic
     flows to the client.
 3.  An attacker attempting to brute-force guess the IPv4 lease
     address and client identifier tuple.  The risk of this can be
     reduced by using a client identifier format that is not easily
     guessable, e.g., by using a random-based client identifier (see
     Section 3.5 of [RFC7844]).
 An attacker could attempt to redirect existing flows to a client
 unable to process the traffic.  This type of attack can be prevented
 by implementing network ingress filtering [BCP38] in conjunction with
 the BR source address validation processes described in [RFC7596]
 Section 5.2 and [RFC7597] Section 8.1.
 A client may attempt to overload the server by sending multiple
 source address update messages (see Section 7.2.1) in a short time
 frame.  This risk can be reduced by implementing a server policy
 enforcing a minimum time interval between client address changes, as
 described in Section 8.1.

9.1. Client Privacy Considerations

 [RFC7844] describes anonymity profiles for DHCP clients.  These
 considerations and recommendations are also applicable to clients
 implementing the mechanism described in this document.  As DHCP 4o6
 only uses DHCPv6 as a stateless transport for DHCPv4 messages, the
 "Anonymity Profile for DHCPv4" described in Section 3 is most
 relevant here.
 In addition to the considerations given in [RFC7844], the mechanism
 that the client uses for constructing the interface identifier for
 its IPv6 softwire source address (see Section 7.1) could result in
 the device being trackable across different networks and sessions,
 e.g., if the client's softwire Interface Identifier (IID) is
 immutable.
 This can be mitigated by constructing the softwire source IPv6
 address as per Section 6 of [RFC7597].  Here, the address's IID
 contains only the allocated IPv4 address (and port set identifier if
 [RFC7618] is being used).  This means no additional client
 information is exposed to the DHCP 4o6 server; it also means that the
 IID will change as the leased IPv4 address changes (e.g., between
 sessions when Section 3.5 of [RFC7844] is implemented).

Farrer, et al. Standards Track [Page 14] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

10. IANA Considerations

 IANA has assigned the OPTION_S46_BIND_IPV6_PREFIX (137) option code
 from the DHCPv6 "Option Codes" registry maintained at
 <http://www.iana.org/assignments/dhcpv6-parameters> as follows:
     Value:             137
     Description:       OPTION_S46_BIND_IPV6_PREFIX
     Client ORO:        Yes
     Singleton Option:  Yes
     Reference:         RFC 8539
 IANA has assigned the OPTION_DHCP4O6_S46_SADDR (109) option code from
 the "BOOTP Vendor Extensions and DHCP Options" registry maintained at
 <http://www.iana.org/assignments/bootp-dhcp-parameters> as follows:
     Tag:          109
     Name:         OPTION_DHCP4O6_S46_SADDR
     Data Length:  16
     Meaning:      DHCPv4 over DHCPv6 Softwire Source Address Option
     Reference:    RFC 8539
 IANA has updated the entry for DHCPv6 OPTION_S46_BR (90) in the
 "Option Codes" registry maintained at
 <https://www.iana.org/assignments/dhcpv6-parameters> as follows:
 Old Entry:
     Value:             90
     Description:       OPTION_S46_BR
     Client ORO:        No
     Singleton Option:  No
     Reference:         [RFC7598]
 New Entry:
     Value:             90
     Description:       OPTION_S46_BR
     Client ORO:        Yes
     Singleton Option:  No
     Reference:         [RFC7598], [RFC8539]

Farrer, et al. Standards Track [Page 15] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

11. References

11.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <https://www.rfc-editor.org/info/rfc2119>.
 [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
            RFC 2131, DOI 10.17487/RFC2131, March 1997,
            <https://www.rfc-editor.org/info/rfc2131>.
 [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
            Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
            RFC 7341, DOI 10.17487/RFC7341, August 2014,
            <https://www.rfc-editor.org/info/rfc7341>.
 [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
            W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
            Configuration of Softwire Address and Port-Mapped
            Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,
            <https://www.rfc-editor.org/info/rfc7598>.
 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
            Richardson, M., Jiang, S., Lemon, T., and T. Winters,
            "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
            RFC 8415, DOI 10.17487/RFC8415, November 2018,
            <https://www.rfc-editor.org/info/rfc8415>.

Farrer, et al. Standards Track [Page 16] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

11.2. Informative References

 [BCP38]    Ferguson, P. and D. Senie, "Network Ingress Filtering:
            Defeating Denial of Service Attacks which employ IP Source
            Address Spoofing", BCP 38, RFC 2827, May 2000,
            <https://www.rfc-editor.org/info/bcp38>.
 [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
            Farrer, "Lightweight 4over6: An Extension to the Dual-
            Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
            July 2015, <https://www.rfc-editor.org/info/rfc7596>.
 [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
            Murakami, T., and T. Taylor, Ed., "Mapping of Address and
            Port with Encapsulation (MAP-E)", RFC 7597,
            DOI 10.17487/RFC7597, July 2015,
            <https://www.rfc-editor.org/info/rfc7597>.
 [RFC7618]  Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q., and M.
            Boucadair, "Dynamic Allocation of Shared IPv4 Addresses",
            RFC 7618, DOI 10.17487/RFC7618, August 2015,
            <https://www.rfc-editor.org/info/rfc7618>.
 [RFC7844]  Huitema, C., Mrugalski, T., and S. Krishnan, "Anonymity
            Profiles for DHCP Clients", RFC 7844,
            DOI 10.17487/RFC7844, May 2016,
            <https://www.rfc-editor.org/info/rfc7844>.

Acknowledgements

 The authors would like to thank Ted Lemon, Lishan Li, Tatuya Jinmei,
 Jonas Gorski, and Razvan Becheriu for their contributions and
 comments.

Farrer, et al. Standards Track [Page 17] RFC 8539 Softwire Provisioning with DHCP 4o6 March 2019

Authors' Addresses

 Ian Farrer
 Deutsche Telekom AG
 Landgrabenweg 151
 Bonn, NRW  53227
 Germany
 Email: ian.farrer@telekom.de
 Qi Sun
 Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-6278-5822
 Email: sunqi.ietf@gmail.com
 Yong Cui
 Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-6260-3059
 Email: yong@csnet1.cs.tsinghua.edu.cn
 Linhui Sun
 Tsinghua University
 Beijing  100084
 China
 Phone: +86-10-6278-5822
 Email: lh.sunlinh@gmail.com

Farrer, et al. Standards Track [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc8539.txt · Last modified: 2019/03/15 22:57 (external edit)